1. Field of the Invention
The present invention relates to a tracking apparatus for a playback apparatus which makes use of a rotational magnetic recording medium, especially when such apparatus is designed to reproduce information signals such as video signals previously recorded on the rotational magnetic recording medium such as magnetic disk.
2. Description of the Prior Art
There has recently been evolved an electronic still camera system which includes an imager such as a solid-state imager or image pickup tube, combined with a magnetic disk of a larger storage capacity functioning as an inexpensive recording medium to electronically shoot a still image of an object on the recording disk, so that the recorded image is reproduced on a separate television system or printer.
The rotational magnetic recording medium used in such an electronic still camera system is a disk with a diameter of about 50 millimeters on which are recorded 50 tracks with a track about equal to 100 .mu.m, that is, with a track width about equal to 50 to 60 .mu.m and a guard band width about equal to 50 to 40 .mu.m. In the recording or reproducing apparatus, the magnetic disk is revolved at a constant speed of e.g. 3,600 r.p.m. to effect a recording or playback of a field or frame of video signals.
The recording medium employed in such magnetic recording is susceptible to tracking error due to compatibility, offset or center deviation, heat expansion or the like with the result that the magnetic reproducing or playback head is apt to sweep the track next to the desired one, thus causing crosstalk.
In order to avoid such a problem, there is also known a system into which the recording head is subjected to a tracking servo to record tracking signals, and the reproducing head is subjected to a tracking servo by making use of these tracking signals. However, it would not be practically advisable to provide the small lightweight recording device such as camera with a tracking servo system which requires a precision control.
There is also known a method wherein the guard band system or an FM azymuth system is used as a recording scheme, and wherein a small amount of tracking error during reproduction is compensated by the arrangement in which the reproducing head is prevented from running on adjacent tracks or, if it does, it is prevented from picking up the signals of these adjacent tracks.
There is also known an envelope peak detecting autotracking control system wherein the recording head is transferred by a stepping motor at a predetermined track pitch without tracking servo, and wherein the reproducing head senses the envelope of the output signals of a track to identify the optimum track on the basis of the peak envelope so as to be subjected to a tracking servo.
For determining the head position at which the envelope shows its peak value, the head is shifted to a predetermined pitch position where the envelope is sensed to be compared to the envelope value obtained at the preceding head position.
For effecting such comparison, a digital processing system is generally used. To this end, the video signals sensed by the magnetic head is envelope detected and the resulting envelope output signal is converted by an analog to digital converter into digital signals, are in turn supplied to a digital processing system. To exclude detection error due to system disturbances, such as noises, only the envelope level in excess of a preset threshold value is advantageously used for such comparison. The dynamic range of the analog to digital converter should be designed so that the envelope level in excess of the preset threshold value is converted, with sufficient resolution, into corresponding digital signals.
With the rotating magnetic recording medium such as magnetic disk steadily revolving at a predetermined r.p.m., the envelope level of the sensed and detected frequency signals is lower for the radially inner track than for the radially outer track, since the radially inner track shows a slower linear speed relative to the head than the radially outer track. In general, for the same frequency range of the frequency signals recorded on the tracks, the envelope level depends on the contact state between the head and the track, the linear speed of the track relative to the head, and the track radius, with the latter being more decisive than the former three factors. Therefore, macroscopically, the envelope output level is lowered in reverse proportion to the track radius. For this reason, the dynamic range of the analog to digital converters is designed by taking the innermost track of the recording medium into account as a reference track.
However, the so-designed analog to digital converters present the problem that the envelope signals derived from the tracks closer to the radially outermost track account for a majority of the dynamic range of the analog to digital converter, whereas the signals derived from the tracks closer to the radially innermost track account for only a minor portion of the dynamic range. This means that the resolution of the envelope signal is sufficiently high with the former tracks, but insufficient with the latter tracks.
More specifically, when the head is shifted with the predetermined pitch as mentioned hereinabove for detecting the envelope peak position, the envelope data converted into the digital value shows different quantum level steps and significant changes in the case of the radially outer tracks, as the head is shifted radially, so that the peak position may be determined positively. In the case of the radially inner tracks, the envelope data is not changed significantly, but shows a rather flat pattern, thus occasionally making it difficult to identify its peak position, or to accomplish tracking with high accuracy.